The generation of electrical signals representing graphic data has been a subject of investigation and study for many years. Applications of developments in the field of electrographics are quite numerous and promising. For example, graphic data in digital form may be treated by computer in providing graphic design problem analysis. Similarly, digitalized graphic information may be stored in computer memory or transmitted between remote stations via telecommunication links.
The generation of electrographic signal is initiated at a man-machine interface which generally is present as a surface upon which graphic data is manually developed. For the most part, such development occurs in the same fashion as graphics are generated utilizing paper, a stylus representing a writing instrument being drawn across the surface to form informational characters or designs. The surfaces upon which this drawing takes place are commonly known as "digitizers." The digitizers respond to the coordinate position of the stylus held by the operator and generate analog coordinate signals which are appropriately treated and converted to digital form for transmission.
For the most part, digitizers have been fashioned as composite structures wherein a grid formed of two spaced arrays of mutually orthogonally disposed fine wires is embedded in an insulative carrier. One surface of this structure serves to receive a stylus input which is converted to coordinate signals. Various methods have been devised for generating coordinate defining signals as a stylus-grid interaction, for example, a magnetostrictive effect may be established between stylus and grid or a capacitive coupling effect may be evoked between these components.
The use of such grid structures, while providing accurate, linear output coordinate signals necessarily involve intricate structures which are expensive to fabricate and prone to damage in the normal course of use. Further, for many applications it is desirable that the digitizer be fabricated as a highly transparent composite sheet. The grid structures within the composite structures, however, militate against achieving such desired transparency.
Another principal approach in the design of digitizers looks to the use of resistive surface coatings. An immediately apparent advantage of this approach resides in the inherent simplicity of merely providing a resistive surface upon a supportive substrate such as glass or plastic. Further, the resistive coating may be transparent to permit an expanded range of industrial applications.
Unfortunately, designers have encountered a variety of technical problems in adopting the resistive layer to provide coordinate output signals. Paramount among these problems has been the non-linear nature of these coordinate read-outs. A precise one-to-one correspondence is required between actual stylus position and the resultant coordinate signals. However, a pin cushion form of distortion, among others, has been encountered by investigators causing the achievement of linearity of output to become an elusive goal. Various forms of correction have been developed; however, each such correction has been at the expense of losing a desired operational attribute or feature of the digitizer. Among these features desired for the digitizer product is a capability of "writing" with the stylus not touching the surface of the digitizer. Additionally, as indicated above, it is desirable that the digitizer be fabricable as a highly transparent surface. Further, it is most desirable that the digitizer work in conjunction with a sheet of opaque paper such that the operator may draw or make positional visual inputs upon the sheet of paper while, simultaneously, the digitizer provides real time coordinate output signals. Next, the structure of the digitizer must remain simple and immune from the wear and related vagaries encountered in common drafting utilization. In the latter regard, where composite structures requiring separation of resistive surfaces followed by flexure of one into the other are evolved, not only the cost of the digitizer becomes elevated but also the operational life and general reliability thereof become compromised.